1. Field of the Invention
The present invention relates to the field of acousto-optic devices and acousto-optic signal processing.
2. Prior Art
Acousto-Optic devices are well-known and widely used light modulators, being generally described in the literature, including Proc. IEEE, Special Issue on Acousto-Optics, Vol. 69, Jan. 1981 and Acousto-Optic Signal Processing: Theory and Implementation, Ed. N. J. Berg and J. N. Lee, Marcel Dekker, Inc., New York, 1983. An input electrical signal s(t) to such a device is converted to a sound field by an input transducer. This wave then travels the length of the crystal. An absorber at the far end of the device causes the wave to terminate with no reflections. The input electrical signal is present on a carrier as s.sub.1 (t)=s(t) cos .omega..sub.c t or s.sub.2 (t)=[B+s(t)] cos .omega..sub.c t, where s(t) is a zero-mean signal and B is a bias. When illuminated with light, the cell diffracts the input light at angles proportional to n.omega..sub.c. These waves are as diffracted orders and the wave .varies..+-..omega..sub.c as the first-order.
As the sound field travels the length of the cell, the sound field s(x,t) in the cell varies in space x and time t. Depending on the Acousto-Optic cell and the input signal s.sub.1 (t) or s.sub.2 (t), the amplitude or intensity of the first-order wave can be made proportional to s(t) or B+s(t) respectively. For amplitude modulation, the input electrical signal is s(t) cos .omega..sub.c t and the amplitude of the first-order wave is EQU A.sub.1 (t,x)=e.sup.j.omega.Lt jA.sub.in Ks(t-x/v)e.sup.j.omega.c(t-x/v) ( 1)
i.e. the amplitude is proportional to s(t-x/v) EQU A.sub.1 (t,x).varies.s(t-x/v), (2)
where K is a constant, A.sub.in is the amplitude of the input light wave and .omega..sub.L is its frequency, and v is the velocity of sound in the Acousto-Optic material. For intensity modulation, the input electrical signal is [B+s(t)] cos .omega..sub.c t and the intensity of the first-order wave is EQU I(t,x)=KI.sub.in [B+s(t-x/v)], (3)
where K is a constant and I.sub.in =A.sub.in.sup.2. Thus, except for a constant bias, the intensity is proportional to s(t-x/v), EQU I(t,x).varies.s(t-x/v). (4)
By a simple change of variables, we can write (2) and (4) as s(x-vt). This latter representation is more appropriate to describe a space integrating Acousto-Optic processor.
The classic time-integrating acousto-optic correlator of FIG. 1 is well-known and described in detail elsewhere, including the two references previously referred to and in R. A. Sprague and C. L. Koliopoulous, "Time Integrating Acousto-Optic Correlator", Applied Optics, Volume 15, pp. 89-92, January 1976; and P. Kellman, "Time Integrating Optical Processors", in Optical Processing Systems, W. Rhodes, ed. (Proc. SPIE, Vol. 185, 1979), pp. 130, 1979. Ignoring Bragg or Raman-Nath mode, amplitude or intensity modulation, any bias and .omega..sub.c carrier, and single-sideband filtering (described in the foregoing references), the operation of the system can easily be described. The system of FIG. 1 consists of a point modulator fed with a signal s.sub.b (t). Its output is expanded (by lens L.sub.1) to uniformly illuminate an acousto-optic cell at P.sub.2. The light distribution incident on P.sub.2 is thus s.sub.b (t), varying in time and being uniform in space. With s.sub.2 (t) fed to the acousto-optic cell, its transmittance is s.sub.a (t-.tau.), where .tau.=x/v as in (2) or (4). The light leaving P.sub.2 is now s.sub.b (t)s.sub.a (t-.tau.). Lenses L.sub.2 image P.sub.2 onto P.sub.3 (and SSB filters the result). Since any bias and the .omega..sub.c carrier have been ignored, the pattern leaving P.sub.2 and the pattern incident on P.sub.3 are the same. The detector at P.sub.3 time integrates the incident pattern and the P.sub.3 output obtained is ##EQU1## i.e the correlation (symbol .circle.* ) of s.sub.a and s.sub.b is displayed as a function of space (.tau..varies.x) at P.sub.3.
The time integrating correlator is advantageous when T.sub.S &gt;T.sub.A and TBWP.sub.S &gt;TBWP.sub.A, where T.sub.S is the signal duration, T.sub.A is the acousto-optic cell aperture time, TBWP.sub.S is the signal time-bandwidth product and TBWP.sub.A is the acousto-optic cell time bandwidth product. The processor of FIG. 1 can thus provide the correlation output for a very long signal, with the integration time T.sub.I of the detector determining the T.sub.S =T.sub.I value used. If detector dynamic range is exceeded, the contents of the detector are dumped and stored (after some T.sub.I '&gt;T.sub.S) and a new integration is begun. By noncoherently adding the R() outputs for separate T.sub.I ', the full T.sub.I =T.sub.S integration is achieved (at a loss of about 3 dB in processing gain due to the noncoherent summation). The time integrating correlator can however only search a limited time delay between signals T.sub.D (-T.sub.A /2&lt;T.sub.D &lt;T.sub.A /2) set by T.sub.A of the acousto-optic cell, i.e., T.sub.D &lt;T.sub.A.